1. Field of the Invention
This invention relates to land mobile radio systems and more specifically to a method and apparatus for minimizing adjacent channel interference by constraining transmitted power envelope variations for mobile radio systems.
2. Description of Related Art
Conventional land mobile radio (LMR) channels employ narrow-band frequency division multiplexed (FDMA) systems for base stations to communicate with mobile radio units. Each base unit/mobile unit communicating pair employs a different frequency band.
Speech communication may be executed by digitizing the speech, and transmitting the encoded speech information. Other digital information may be desired to be communicated between a base and mobile radio unit.
Currently, there is an immediate need for an increase in capacity of LMR systems in the U.S. for such applications as public safety trunking. The trend is to increase capacity by splitting each existing 25 KHz channel used in LMR systems into two 12.5 KHz channels. However, this causes adjacent channel interference (ACI). ACI is interference introduced at a receiver from a transmitter broadcasting at a frequency corresponding to an adjacent channel and this is sometimes called adjacent channel `splatter`. This interference is increased by intermodulation and harmonic distortions caused by non-linearities in a radio frequency (RF) amplifier in the transmitter.
The need for capacity in land mobile radio (LMR) systems can be met by improving spectral efficiency (to values greater than 1 bit/sec/Hz). However, the achievable spectral efficiency is limited by stringent adjacent-channel interference protection specifications.
Continuous phase modulation (CPM) schemes such as 4-level digital FM can meet the adjacent channel interference protection ratio (ACIPR) requirements and also exhibit acceptable distortions from a non-linear RF amplifier. However, these schemes offer inadequate spectral efficiency (0.75 bits/sec/Hz) to meet future increases in capacity.
Linear modulation, such as .pi./4-shifted differential quadrature phase shift keying (DQPSK), offers higher spectral efficiency than constant envelope CPM schemes but requires highly linear RF amplifiers to suppress ACI. The requirement of a linear RF amplifier arises due to high envelope variations exhibited by .pi./4-shifted DQPSK. Filtered .pi./4-shifted DQPSK offers an ACIPR of approximately 40-45 dB with a linearized conventional class-AB power amplifier as described in "Highly Efficient Digital Mobile communications with a Linear Modulation Method" by Y. Akaiwa and Y. Nagata, IEEE Journal of Sel. Areas in Commun., vol SAC-5 no. 5, pp. 890, June 1987 and U.S. patent application, "Improved Trellis Coding Technique to Increase Adjacent Channel Interference Protection Ratio in Land Mobile Radio Under Peak Under Peak Power Constraints", by S. Chennakeshu, et al, Ser. No. 07/975,201 filed Jun. 15, 1992.
The above comparison of constant envelope and linear modulation schemes is based on power and bandwidth occupancy. However, it is also important to address decoding complexity for the two schemes. While linear modulation schemes require amplifier linearization, these schemes are easier to synchronize and equalize for intersymbol interference (ISI) relative to continuous phase modulation schemes.
Currently there is a need for a digital mobile radio system exhibiting a linear modulation scheme which meets the stringent specifications on ACIPR with relaxed linearity requirements on the RF amplifier.